The sleep apnea-hypopnea syndrome (SAHS) is characterized by repetitive complete (apnea) or partial (hypopnea) collapse of the upper airway during sleep. Apnea events are associated to hypoxemia, heart rate variations and arousals. Epidemiological data support the finding that SAHS may have a role in the initiation or progression of diverse respiratory, cardiovascular, and cerebrovascular diseases. The incidence of SAHS has been estimated at 5% of adults in western countries. Furthermore, previous studies revealed that a high percentage of patients (82% of men and 93% of women) with moderate or severe SAHS might remain undiagnosed. Therefore, early detection and treatment of SAHS are required in order to prevent long-term effects and end-organ damages.
Nowadays, nocturnal polysomnography (PSG) is the gold-standard for SAHS diagnosis. PSG studies are performed in special sleep units and generally involve monitoring several physiological recordings such as electrocardiograms (ECG), electroencephalograms (EEG), electromyograms (EMG), electrooculograms (EOG), airflow signals, respiratory effort, and oxygen saturation (SaO2) or oximetry. These signals are typically manually analyzed by a sleep specialist in order to identify every episode of apnea/hypopnea. The number of detected events is divided by the hours of sleep to compute the apnea-hypopnea index (AHI), which is used to assess SAHS severity. However, PSG studies have drawbacks since they are costly, time-consuming, and require subjects to be overnight in a special medical facility. Additionally, the demand for PSG studies is progressively growing as people and clinicians are becoming aware of SAHS whereas the available infrastructure is insufficient to support it. Consequently, new methods, apparatuses and systems focused on alternative diagnostic methods that overcome some of the limitations associated to PSG are needed.
Related art includes U.S. patent application Ser. No. 10/947,983 which discloses a method for diagnosing OSAS based on a tool for the predicting Apnea Hypopnea Index (AHI) using non-parametric analysis and bootstrap aggregation, and U.S. patent application Ser. No. 11/122,278 which discloses a method for monitoring respiration involving processing plethysmography signals.
New techniques for simplified SAHS detection have been commonly based on the analysis of a reduced set of data. The utility of clinical and demographic variables, as well as ECG has been studied by the research community. In the context of this problem, SaO2 signals recorded through nocturnal pulse oximetry are of special interest since they can be easily acquired and enable for portable monitoring. Pulse oximetry is a non-invasive technique used to monitor arterial blood oxygen saturation. Oximetry recordings contain essential information about SAHS and play a crucial role to interpret PSG studies. Apneas and hypopneas are usually accompanied by marked desaturation events due to the lack of airflow. As a result, patients with SAHS tend to present unstable SaO2 signals due to frequent drops in the saturation value. A different behavior tends to be observed in healthy patients. Their recordings reflect normal ventilation, which corresponds with a saturation value near 90% and the absence of repetitive abrupt changes in the SaO2 profile.
Improved methods and the corresponding apparatuses are still needed in order to provide more accurate models that characterize SAHS from SaO2 signals, especially to determine the severity of SAHS.